The invention relates to an electronic assembly with a printed circuit board on which electronic components are mounted and a metal plate having a first and second surface and where the printed circuit board is arranged with a positive or direct connection on the first surface of the metal plate.
Assemblies for electronic components, in particular for automotive electronics, must satisfy increasingly high demands in terms of function, quality and reliability. As regards economy, however, these demands are exposed to certain limits. Consequently, in the automotive field in particular, those technologies are used that on the one hand can satisfy the technical requirements and on the other hand permit cost-efficient manufacture.
From DE-OS 41 02 265 an assembly for automotive electronics is known in which a metal plate is used as a carrier for a multilayer printed circuit board or a multilayer foil, and at the same time act as a heat sink for the components located on the printed circuit board or foil as well as being part of the if housing. The assembly becomes a complete housing by providing a hood-shaped cover that positively covers the side of the metal plate that carries the printed circuit board or foil. Furthermore, the space between the cover and the printed circuit board can be filled with an insulating and moisture-resistant material while forming a molding. In this known assembly, a critical point continues to be the hermetically sealed packaging of the components as well as the printed circuit board because leakages can occur between the housing cap, which is generally made of a plastic material, and the metal plate. One measure to improve reliability would be to manufacture a full or partial molding as a soft molding by filling silicon gel in the hollow space between housing cap and printed circuit board. However, because of the high material cost of silicon, this results in high production costs. Furthermore, the high thermal coefficient of expansion of silicon gel leads to mechanical problems.
The object of the invention is to provide an assembly of the type named at the outset that offers maximum reliability with regard to the sealing of the components and the printed circuit board and low manufacturing costs at the same time.
According to the invention there is provided an electronic assembly with a printed circuit board on which electronic components are mounted and a metal plate having a first and second surface, wherein the printed circuit board is located positively (directly) on the first surface of the metal plate, this metal plate having a surface that overhangs the periphery of the printed circuit board, and where a molding of the is components and the printed circuit board is provided, and where this molding provides a positive or direct connection with the overhanging part of the metal plate at the periphery of the printed circuit board.
For the first time in assembly technology, the present invention makes use of the housing technique known from IC technology, namely that of making the housing as a hard molding by surrounding the IC chips with a mold mass. The molding provides a positive connection at the periphery of the printed circuit board with the metal plate so that together with the metal plate this molding forms a high quality moisture-proof housing that is very reliable even under conditions of cyclic thermal loading.
In accordance with an advantageous embodiment, the molding is made in such a way that the metal plate overhangs at the periphery of the circuit board and molding, thus providing the means of attachment at the overhang.
As a rule, electronic assemblies have the means for connecting leads to them, that is by means of plug connectors, which are joined to the printed circuit board. Such connectors comprise a plug core, which holds the plug pins, and a plug frame which serves as a guide socket and which is provided with a seal and latch. In an embodiment of this type, the plug frame as a molding around the plug core is at the same time made as a molding around the components and the printed circuit board. The plug costs can thus be minimized because the production costs of the plug manufacturer are reduced and in total this leads to lower production costs for the assembly. In an embodiment of this type, the printed circuit board is located on the first surface of the metal plate, while a plug of this type is located on the second surface of this metal plate and is enclosed on both sides by the molding mass.
In another preferred development of the invention, the plug is designed in the form of an edge connector. For fitting an edge connector of this type, the metal plate has a recess in its edge region, with this recess being so deep that an edge region of the printed circuit board sufficient for fitting the edge connector extends into this recess, while this edge region of the printed circuit board is not enclosed by the molding mass. Since the plug itself is on the cable side in such an embodiment, the assembly has no separate plug Which means that the housing tool costs for plug, cover etc. are reduced to just a single tool.
In such an embodiment, on the second surface of the metal plate in the edge region of the printed circuit board accommodating the edge connector, a surrounding molding of the contact area of the metal plate with the printed circuit board is preferably provided in order to prevent the penetration of moisture.
Moreover, the edge connector in the above embodiment can advantageously be designed as a zero-force plug connector.
A further reduction in production costs is obtained by using components without housings, i.e. IC chips, instead of components with housing, and which are bonded to the printed circuit board after soldering. The main reason for the cost saving is that the semi-conductor manufacturer no longer needs to manufacture the housing.
Finally, in a last advantageous development of the invention, a multilayer printed circuit board or a multilayer foil can be used as the printed circuit board, which results in better utilization of the existing printed circuit board surface and a consequently smaller volume of the assembly.